Pyrazolinone derivatives such as 1-[(2-propenylthio)carbonyl]-4-(2-methylphenyl)-5-amino-1H-pyrazol-3-one are known to be effective for controlling plant diseases. As a method for purifying the derivative, a method for purifying a mixture containing the derivative by column chromatography is described in Patent Document 1.    [Patent Document 1] Japanese Unexamined Patent/Application Publication No. 2000-226374